Naphthalenedicarboxylic acid is used to make extrusion and injection-molding resins because of the good heat resistance, high glass transition temperature, and gas barrier properties of naphthalenedicarboxylic acid based polymers. Polymers containing naphthalenedicarboxylic acid are used in the fabrication of various articles for household or industrial use, including appliance parts, containers, and auto parts. One major drawback of naphthalenedicarboxylic acid containing polymers, however, is their inherent bluish fluorescence. Thus, objects prepared with naphthalenedicarboxylic acid containing polymers have a hazy and bluish appearance. This phenomenon is especially of concern in the packaging of foods and beverages wherein the food or beverage inside a container made from a naphthalenedicarboxylic acid containing polymer appears unnatural.
The fluorescence of homopolymers of poly(ethylene 2,6-naphthalenedicarboxylate), referred to as PEN, is known in the art. Because of the improved properties of naphthalenedicarboxylic acid containing polymers, it is desirable to incorporate small amounts of naphthalenedicarboxylic acid in polymers such as poly(ethylene terephthalate) (PET). However, copolymers containing very small amounts of naphthalenedicarboxylic acid fluoresce with intensity similar to, or in some cases greater than PEN homopolymers. Surprisingly, poly(ethylene terephthalate) modified by copolymerizing in less than 1 mole percent naphthalenedicarboxylic acid has significant visible fluorescence.
Fluorescence is a type of luminescence in which an atom or molecule emits radiation in passing from a higher to a lower electronic state. The term is restricted to phenomena in which the time interval between absorption and emission of energy is extremely short (10.sup.-10 to 10.sup.-6 second). Fluorescence in a polymer or small molecule, occurs when a photon is emitted from an excited singlet state. Quenching of fluorescence eliminates or reduces the ability for photon emission by providing an alternative pathway for the excited state energy such as vibronic or heat loss, or intersystem crossing to the excited triplet state.
Methods to quench fluorescence in PEN have been disclosed by Chen Shangxian et al. in an article entitled, "Fluorescence Spectra of Poly(Ethylene-2,6-Naphthalene Dicarboxylate)" which appeared in SCIENTIA SINICA, Vol. XXIV, No. 5, May 1981, and by CAO Ti et al. in an article entitled, "Intermolecular Excimer Interaction In Poly(Polytetramethylene Ether Glycol Aryl Dicarboxylate)" which appeared in ACTA CHIMICA SINICA, Vol. 42, No. 1, 1984. Both of the references disclose the use of o-chlorophenol to quench PEN fluorescence in a chloroform solution. Dissolving the PEN in a chloroform solution to disperse the fluorescence quencher therein, however, is not practical on an industrial scale because only very dilute PEN solutions can be prepared. In addition, the PEN must have a low molecular weight to dissolve in the chloroform solution.
In contrast, the present inventors have unexpectedly determined that melt blending a naphthalenedicarboxylic acid containing polymer with 0.1 to 5.0 weight percent of a fluorescence quenching compound selected from a halogen containing aromatic compound, an aromatic ketone and a naphthol compound, provided said fluorescence quenching compound contains an aromatic ring having at least one acyl group, halogen atom or hydroxyl group directly attached to the aromatic ring, significantly reduces the fluorescence of the polyester without deleteriously effecting the physical properties of the polyester.